Input common mode voltage is stated in Table 2 of the datasheet. All of Table 2 is for VREF = 1.4V.

For VREF = 1.4V:

The minimum input common mode voltage is 0.7V.

The maximum input common mode voltage is 1.0V.

The nominal input common mode voltage is 0.9V.

With VREF = 1.4V, the full-scale differential input voltage is 2.8Vpp_differential. This means that each side of the differential signal swings 1.4Vpp; 0.7V below VCM and 0.7V above VCM. As a result, the maximum voltage for the input signal (with respect to ground) would be VCM + 0.7V = 0.9V + 0.7V = 1.6V (with nominal VCM and a full scale signal). Similarly, the minimum voltage for the input signal (with respect to ground) would be VCM - 0.7V = 0.9V - 0.7V = 0.2V (with nominal VCM and a full scale signal).

Input common mode voltage is stated in Table 2 of the datasheet. All of Table 2 is for VREF = 1.4V.

For VREF = 1.4V:

The minimum input common mode voltage is 0.7V.

The maximum input common mode voltage is 1.0V.

The nominal input common mode voltage is 0.9V.

With VREF = 1.4V, the full-scale differential input voltage is 2.8Vpp_differential. This means that each side of the differential signal swings 1.4Vpp; 0.7V below VCM and 0.7V above VCM. As a result, the maximum voltage for the input signal (with respect to ground) would be VCM + 0.7V = 0.9V + 0.7V = 1.6V (with nominal VCM and a full scale signal). Similarly, the minimum voltage for the input signal (with respect to ground) would be VCM - 0.7V = 0.9V - 0.7V = 0.2V (with nominal VCM and a full scale signal).

If you go to the AD9655 product page on www.analog.com, and scroll down to "AD9655 Input Impedance", you will find more information on the simplified lumped parallel and lumped serial representations.

In "AD9655 Input Impedance" there is a mistake in one of the formulas that causes the capacitance values to appear as negative values. Please ignore the minus signs. I'll be working on getting this fixed.

You could use the lumped parallel model OR the lumped serial model, but not both.

The lumped serial model is 30Ohms in series with 6.7pF (as shown in the "AD9655 Input Impedance" file, in the series data). In this case you would not have the shunt resistor.

The lumped parallel model is 1.9kOhms in parallel with 6.6pF (as stated in datasheet Table 1 and Table 2), with no series resistors. This is also found in the "AD9655 Input Impedance" file, in the parallel data. The 1.9kOhms || 6.6pF values are for 100MHz (can be seen in the "AD9655 Input Impedance" file), but produce reasonable impedance numbers for a much wider band..

Some applications favor use of the lumped parallel model, but if your signal is very wide band, or you do not care which model to use, I recommend you use the lumped series model.